Rechargeable batteries are of great importance to many industrial sectors such as the electrical power industry and automotive industry. One class of such batteries is lead-acid based batteries. A conventional lead-acid battery has a series of battery cells with separate monopolar positive and negative electrode grids that are suspended in an acid electrolyte solution. Each electrode usually comprises a grid made of lead or a lead alloy that is covered or filled with positive or negative active materials. Lead or a lead alloy are widely used to take advantage of its resistance to corrosion, low cost, and high gassing over-voltage for both hydrogen and oxygen.
Two adjacent cells are electrically connected through conductors that join the positive electrode grid of a cell to the negative electrode grid of the other cell or vice versa.
The electric current in an electrode grid flows in parallel to the plane of the grid and enters an adjacent cell through the connecting conductors. Batteries with such construction have limits that are related to several factors inherent in their construction.
The excessive weight required for both high-power generation and energy storage impedes many applications. The conventional construction of an electrode grid makes a long electrical path between cells and small electrical cross-section. This forces the electric current to travel along a high resistance path and, therefore, may limit the useful output power from the battery. Such batteries often have limited life, at least partly because of the degradation of the positive active material and the corrosion of the lead material used to construct the positive grids.
A bipolar lead-acid battery shown in FIG. 1 uses a different approach. An external case 100 encloses a series of parallel bipolar plates 104. Each bipolar plate 104 is a solid sheet, with its positive side covered by positive active material 106 and its negative side covered by negative active material 108. Bipolar plates 104 partition the battery into a series of cells and also provide electrical contact between the positive and negative electrode materials of adjacent cells. Each cell has a separator 112 which is filled with sulfuric-acid electrolyte. The separator 112 is usually made of glass mat material, in which the electrolyte is absorbed. Two end plates 110 are connected to the positive terminal 101 and the negative terminal 102.
One distinct advantage of such a bipolar battery is that the direction of the electric current is perpendicular to the bipolar battery plates 104. The cross-section of the current flow is, therefore, entire bipolar plate 104. This cross-section is usually much larger than the cross section of the grids of conventional lead-acid batteries. In addition, the electrical path in such a bipolar battery is significantly reduced because of the thin bipolar battery plates. As a result, the intra-cell resistance is minimized and a relatively low overall resistance is achieved with a typical value of approximately one-half to one-fifth of that in a conventional monopolar battery. This facilitates higher power battery sources to become possible.
However, there have been some critical technical problems that have limited the practicality of bipolar batteries. For example, unlike grids of a monopolar battery, the bipolar plates 104 must simultaneously withstand both the positive and negative electrochemical reactions: oxidation and reduction. One major technical challenge in developing a high-power bipolar lead-acid battery has been the construction of a bipolar plate that is light-weight, but that does not achieve the reduced weight by adding more cost or by compromising power (i.e., electrical resistance) or useful lifetime. Various designs on the bipolar plate have been proposed, such as composite bipolar plates described in U.S. Pat. No. 4,658,499 and U.S. Pat. No. 4,507,372. A major improvement in bipolar battery plate construction is disclosed in U.S. patent application Ser. No. 08/096,118, entitled "Battery Plates with Lightweight Cores". The present invention can utilize either a conventional lead/lead alloy bipolar plate or a lightweight bipolar plate that has a long lifetime, high resistance to acidic erosion and oxidation, and reduced manufacturing cost.
Parallel to the progress in bipolar plate technology, the overall construction of a bipolar battery has also been continuously improved. However, some technical problems still remain unsolved in bipolar battery construction.
One technical problem is the electrolyte leakage between battery cells in prior bipolar lead-acid batteries. This leakage of electrolyte can cause an electrical short circuit between the positive active materials and negative active materials of adjacent cells. This leakage may cause a leakage current which can self-discharge the battery cells.
The performance of a bipolar battery is also reduced by the non-uniform separator pressure associated with the rigid plastic seals used in prior battery construction. These seals provide a gas-tight seal between each cell to prevent escape of water vapor and gas, leakage of electrolyte liquid, and entrance of O.sub.2 gas, which may discharge the negative plate. The seals also maintain a specific spacing between bipolar plates. Such gas-tight sealing on every cell is difficult to achieve in production and is also problematic to maintain during the lifetime of a battery due to inevitable changes in the dimension of active materials and separators. Also, each cell has typically been individually sealed with a dedicated venting device implemented in every cell. This is not only cumbersome but also complicates the manufacturing operation. Moreover, maintaining a high clamping pressure on the separator is advantageous to battery performance as explained herein. Rigid seals which have been used for this purpose have limited the clamping pressure that can be applied to the separator.
The inventors of the present invention have found that optimal battery performance is achieved by maintaining a high uniform, separator pressure of optimum value. A high uniform pressure on the positive active material is important. The life of this electrode apparently increases when particle-to-particle contact in the positive active material is well maintained and the amount of this pressure changes over time. Various techniques based on material expansion have been developed to increase and maintain the pressure on the positive active material in other types of lead-acid batteries. Examples of such efforts include the use of tubular positive plates to confine the positive active material and the use of various clamping mechanisms to confine material expansion.
In recognition of these and other limitations in the prior art, the present specification describes an improved bipolar lead-acid battery. More specifically, the present invention describes a "cup" design in bipolar plates, a novel lip or rim type seal for individual battery cells, spring conductors used both for maintaining pressure and providing electrical conduction, simplified current collecting plates, a single battery-venting device, and several other new features. The present invention defines additional elements beyond those described in U.S. patent application Ser. No. 08/161,970, "Bipolar Battery Construction", which become apparent in view of the detailed description of a preferred embodiment herein.
The inventors of this invention recognized the importance of the sealing means of a bipolar battery in preventing cells from self-discharging by increasing the path length between two adjacent cells without significantly increasing the physical dimensions of the battery, in minimizing the leakage current should the electrolyte leak, and in maintaining a gas-tight seal in the battery cell stack. The inventors recognized the desirability of implementing a venting function in the sealing means so that a internal positive pressure can be released in case of excess gassing.
The inventors recognized that the integration of a uniform and high compression force on battery separators by a spring-loaded clamping device can increase lead-acid battery life, and increase the power of the battery stack by maintaining optimal pressure between the bipolar plates in each bipolar cell.
The inventors further recognized, importantly, that the end plates in a bipolar battery can be constructed in a similar way to the bipolar plates, to reduce the manufacturing cost and process. The current collection can be accomplished by using the spring-loaded clamping device to simplify the battery construction.
Automotive and electrical power industries have been making continuous effort in high-power batteries such as bipolar batteries. The inventors of the present invention recognized the commercial need for a practical bipolar battery technology and propose significant improvements in several important aspects of the bipolar battery construction including long lifetime, simplified manufacturing, and reduced manufacturing cost. Implementation of the present invention makes it possible to use bipolar battery technology in battery-powered electrical vehicles and other high-power storage applications.